1. Field of the Invention
The present invention relates to an apparatus for detecting turn-on state of plural heaters, and more particularly, to an apparatus for detecting turn-on state of plural heat lamps, for example, provided in a thermal fixing unit of a copying machine.
2. Description of the Prior Art
Conventionally, there has been widely used a heat roller type thermal fixing unit in apparatuses such as electrophotographic printers and copying machines. The heat roller type thermal fixing unit comprises a pair of rollers, each roller including heat lamp therein as heating sources for the roller, and a sheet of copying paper on which a toner image has been transferred is passed between a pair of rollers so that the toner image is thermally fixed on the copying paper.
Meanwhile, a pair of heat lamps are controlled so as to be turned on and off in association with an copying operation of the copying machine. In this case, there is an optimum temperature or a range of temperature for fixing a toner image thereon, and a pair of heat lamps are automatically controlled to be turned on or off so as to maintain the optimum temperature or the optimum range of temperature (See, for example, U.S. Pat. No. 4,737,818). In order to control a pair of heat lamps to be turned on or off, there is provided a detection device for detecting whether a pair of heat lamps is really turned on or off based on the automatic control device.
In the conventional detection device, turn-on state of each of the heat lamps is individually detected by respectively detecting currents supplied thereto, or by detecting infrared lights emitted from filaments thereof.
Accordingly, since turn-on or off state of each of the heat lamps is individually detected in the conventional detection device, it is necessary to provide detection circuits for respective heat lamps. As a result, the conventional detection device can not be miniaturized.